PTFE fiber is used in numerous demanding applications due to the robust physical properties of PTFE. It has excellent high and low temperature performance, chemical resistance, and resistance to damage as a result of exposure to solar ultraviolet radiation. Exemplary applications of PTFE fiber include use as a dental floss, a bearing, and fabric formed from a wide array of textile processes including weaving, braiding, knitting and needlepunching. PTFE fiber is also used as a sewing thread.
PTFE sewing thread is typically produced by forming or acquiring a PTFE fiber, twisting it longitudinally to obtain a pseudo-round cross section, and then exposing the twisted fiber to elevated temperatures in order to permanently set the twist in the fiber. PTFE fiber can be produced through various methods. These methods include multi-filament emulsion spinning (such as described in U.S. Pat. No. 2,772,444), paste extruding and expansion (such as described in U.S. Pat. No. 3,953,566), and paste extruding and melt-stretching (such as described in U.S. Pat. No. 5,167,890).
Sewing thread manufactured by any of the above methods is typically difficult to sew. Sewing thread made from emulsion-spun multi-filament PTFE, for example, is approximately one-half or less as strong as other PTFE sewing threads. As a result, thread breaks during sewing with emulsion spun PTFE are very common. This material is also only available in brown or white (bleached brown).
Sewing thread made from either of the paste extrusion methods, while stronger than emulsion-spun material, is also difficult to sew, particularly as compared to common polyester sewing thread. Care should be taken when setting-up a sewing machine to handle PTFE fiber. Of particular importance are thread tension adjustments, machine timing, thread lubrication and needle selection. Even when these steps are taken, breaking of PTFE sewing thread may occur. Breaks usually occur more frequently per linear distance as the sewing speed (stitches per minute) is increased.
Polyester sewing thread is robust in sewing due in part to the mechanical properties of the thread. In particular, the "toughness" (as defined herein) of the polyester thread appears to be much higher than that of typical PTFE threads.
The sewing process generates relatively high frequency (greater than about 1000 cycles per minute) tensile loading on the sewing thread. As the sewing speed is increased, the frequency of this loading correspondingly increases. As used herein with reference to sewing, "high speed" means 1500 stitches per minute or greater on a lockstitch sewing machine.
When the sewing thread is subject to tension during the sewing process, it reacts by both elastic and inelastic deformation (temporary and permanent stretching). Without being limited by theory, it is possible that the polyester sewing thread breaks much less often than PTFE sewing thread run under comparable conditions because of the higher level of toughness of the polyester. The increased toughness allows the polyester sewing thread to more readily absorb the tensile loading during sewing through stretching of the sewing thread.
A PTFE fiber having increased toughness is desirable in many applications. In particular, a PTFE fiber adapted to being sewn at high speed is desirable.